1. Field of the Invention
The present invention, generally, relates to a lithography process for semiconductor fabrication. More particularly, the present invention relates to multi-exposure lithography methods and systems that provide improved overlay accuracy.
2. Description of Related Art
To perform a lithography process for manufacturing semiconductor devices having a stack structure, the overlay between a preformed lower layer and an upper layer must be checked. As semiconductor devices become highly integrated and reduced in size, the accuracy of the overlay between the lower layers and the upper layers becomes increasingly more important to improve the reliability and yield of the semiconductor devices.
Generally, the overlay is typically checked by using overlay marks as shown in FIG. 1. Referring to FIG. 1, an overlay mark 10 comprises a frame-shaped first mark 20, which is formed when a lower layer is formed in a previous process of a test wafer, and a plate-shaped second mark 30, which is formed in a subsequent process. In general, the overlay mark 10 is arranged in a scribe line that separates dies. However, when one field is formed of four dies, the overlay mark 10 is arranged at each corner of a field along the outermost scribe lines. In this case, the first mark 20 is formed by patterning the lower layer so that the first mark 20 is formed of the same material as the lower layer. The second mark 30 is formed when patterning a photoresist, i.e. the second mark 30 is formed of photoresist.
Measuring equipment is used to read X-axis direction gaps X1 and X2 between the first and second marks 20 and 30. Thereafter, an offset X of the second mark 30 is calculated using the measured X-axis direction gaps X1 and X2. Here, the offset X of the second mark 30 is a deviation value of the center of the second mark 30 from the center of the first mark 20. The offset X of the second mark 30 is calculated by subtracting the value X2 from the value X1 and dividing the subtraction value by 2. As the offset X approaches zero, the overlay becomes more accurate. Since the first and second marks 20 and 30 may shift in a Y-axis direction, an offset in the Y-axis direction may be measured after measuring the offset in the X-axis direction.
The measuring equipment provides various overlay parameters, including the offsets in the X-axis and Y-axis directions, to the exposure equipment. The overlay for subsequent wafers for forming actual semiconductor devices is corrected based on the measurement of the overlay parameters and an exposure process is performed.
FIG. 2 is a flowchart of the above-described exposure method. Referring to FIG. 2, photoresist is coated on a wafer (step 100), and a photomask and the wafer are aligned (step 110). Thereafter, an overlay is corrected using the measured overlay parameters that are input to the exposure equipment (step 120). Then, images are exposed (step 130), and the exposed photoresist is developed (step 140).
Improvements to the exposure equipment and lithography methods make it possible to manufacture devices of a small pitch having fine patterns. However, improvements to the exposure equipment typically do not match the demands of the semiconductor device manufacturers that desire to form finer patterns. As such, an illumination system can be used which is optimized for the patterns of specific shapes, e.g., an off-axis illumination or a small sigma conventional illumination. In other cases, photoresist, which is specialized for a contact hole, or line and space, may be used to meet to the demands of semiconductor device manufacturers.
Since semiconductor devices include various patterns on a single layer, existing exposure equipment and methods are not diverse enough to process the various patterns that may exist for highly integrated devices. Improving the exposure equipment is time consuming and costly with respect to the manufacture of the semiconductor devices. Therefore, it is advantageous to solve the above problems by focusing on the process, for example, using a multi-exposure lithography method.
In general, a multi-exposure lithography method is performed by separating the layout of one layer into two or more sub-layouts or sub-images according to shape, size, and pattern arrangement, and subsequently exposing the sub-layouts. Then, the multi-exposure method is completed by developing the layout on the layer. Each of the sub-layouts (or the sub-images) may be arranged on one photomask or a plurality of photomasks.
FIG. 3 illustrates various layouts that may be used for a multi-exposure lithography method. A layout 50 can be arranged on one photomask 40 and patterned by performing a single exposure process. However, to perform a multi-exposure method, the layout 50 can be divided into two sub-layouts 50a and 50b. The sub-layouts 50a and 50b can be arranged on one photomask 60 to perform the exposure process twice (i.e. once for each of the sub-layouts 50a and 50b). In another case, each of the sub-layouts 50a and 50b may be arranged on two separate photomasks 70a and 70b, respectively. When a multi-exposure lithography method is performed, the process latitude of exposure latitude (EL) and depth of focus (DOF) can be attained by using exposure conditions optimized for each sub-layout pattern.
Conventional multi-exposure lithography methods will now be described with reference to FIGS. 4 through 6. FIG. 4 is a flowchart of a conventional multi-exposure method, in which a plurality of sub-layouts or sub-images are arranged on one photomask. Referring to FIG. 4, a photoresist is coated on a wafer (step 200), and then the photomask and the wafer are aligned (step 210). Thereafter, an overlay is corrected using an input overlay parameter (step 220). Next, one sub-image is exposed according to the corrected overlay (step 230). Then, it is determined whether all sub-images are exposed (step 235). If another sub-image to be exposed exists, then the remaining unexposed sub-image is exposed using the same input overlay parameter (step 230). When all of the sub-images are exposed, the exposed resist is developed (step 240).
FIG. 5 is a flowchart of a conventional multi-exposure method, in which a plurality of sub-layouts or sub-images are arranged on a plurality of photomasks. Referring to FIG. 5, photoresist is coated on a wafer (step 300), and then a photomask and the wafer are aligned (step 310). Thereafter, an overlay is corrected using an input overlay parameter (step 320). Next, one sub-image is exposed according to the corrected overlay (step 330). Then, it is determined whether all sub-images are exposed (step 335). If a sub-image to be exposed exists, another photomask (having a next selected sub-image) and the wafer are aligned (step 310) and the overlay is corrected using the same input overlay parameter (step 320). The sub-image is exposed according to the corrected overlay (step 330). When all of the sub-images are exposed (affirmative determination in step 335), the exposed photoresist is developed (step 340). Although a correction process is performed after changing the photomasks in FIG. 5, the overlay parameter of the correction is not changed according to the characteristics of each sub-layout.
In the conventional methods, the input overlay parameter that is used when performing the overlay correction is preselected based on desired criteria. For instance, the overlay parameter of a sub-layout, whose overlay conditions should be managed most tightly, can be measured and fed back to the exposure equipment to be used as the overlay parameter. However, as the overlays of every pattern become important due to reduction of the design rule of the devices, the above-described method becomes inappropriate. In another case, the input overlay parameter can be determined by measuring overlay values between lower and upper layers for each sub-layout, and feeding back an average of the measured values to the exposure equipment to be used as the overlay parameter. This method represents calculating the average overlay offset of a plurality of the sub-layouts. However, as shown in FIG. 6, when each sub-layout has different registrations 85a and 85b for a field registration 80 of the lower layer, the correction according to the average value generates an uncorrectable residual term 85 for each sub-layout. Accordingly, the method of using the average value of the overlay offsets to determine an overlay parameter is inappropriate.
Regardless of the arrangement of a plurality of sub-layouts or sub-images on one photomask or on a plurality of photomasks, each image cannot have the same field registration. Consequently, methods for providing increased accuracy of the overlay according to the sub-layout in a multi-exposure lithography method and process would be highly desirable.